Superoxide anion (O2-) generation in response to the chemoattractant f-met-leu-phe was measured in PMNL from 30 patients with ankylosing spondylitis and in PMNL from 27 controls. At a concentration of stimulant above that necessary to achieve maximal response, no differences in O2- production between the groups were noted. With a lower concentration of stimulant, cells from patients with AS or RA taking non-steroidal anti-inflammatory drugs had lower levels of O2- generation, compared both to controls and to AS patients, off medication. Results suggest any decrease in O2- generation seen in patients with AS is likely to be related to therapy and not to the disease or to possession of HLA B27.